FIG. 1 shows major functional blocks in the signal processing arrangement typical of existing cochlear implant (CI) systems wherein band pass signals containing stimulation timing and amplitude information are assigned to stimulation electrodes. Preprocessor Filter Bank 101 pre-processes an initial acoustic audio signal, e.g., automatic gain control, noise reduction, etc. Each band pass filter in the Preprocessor Filter Bank 101 is associated with a specific band of audio frequencies so that the acoustic audio signal is filtered into some N band pass signals, B1 to BN where each signal corresponds to the band of frequencies for one of the band pass filters.
The band pass signals B1 to BN are input to a Stimulation Pulse Generator 102 which extracts signal specific stimulation information—e.g., envelope information, phase information, timing of requested stimulation events, etc.—into a set of N stimulation event signals S1 to SN, which represent electrode specific requested stimulation events. For example, channel specific sampling sequences (CSSS) may be used as described in U.S. Pat. No. 6,594,525, which is incorporated herein by reference.
Pulse Mapping Module 103 applies a non-linear mapping function (typically logarithmic) to the amplitude of the each band-pass envelope. This mapping function typically is adapted to the needs of the individual CI user during fitting of the implant in order to achieve natural loudness growth. This may be in the specific form of functions that are applied to each requested stimulation event signal S1 to SN that reflect patient-specific perceptual characteristics to produce a set of electrode stimulation signals A1 to AN that provide an optimal electric representation of the acoustic signal.
The Pulse Mapping Module 103 controls loudness mapping functions. The amplitudes of the electrical pulses are derived from the envelopes of the assigned band pass filter outputs. As shown in FIG. 2, a logarithmic function with a form-factor C typically may be applied to stimulation event signals S1 to SN as a loudness mapping function, which generally is identical across all the band pass analysis channels. In different systems, different specific loudness mapping functions other than a logarithmic function may be used, though still just one identical function is applied to all channels as shown in FIG. 2 to produce the electrode stimulation signals A1 to AN outputs from the Pulse Mapping Module 103.
Finally, patient specific stimulation is achieved by individual amplitude mapping and pulse shape definition in Pulse Shaper 104 which develops the set of electrode stimulation signals A1 to AN into a set of output electrode pulses E1 to EN to the electrodes in the implanted electrode array which stimulate the adjacent nerve tissue.
Looking more closely at the operation of the Pre-Processor Filter Bank 101, CI signal processing seeks to imitate the natural behavior of a normal ear. A pre emphasis filter typically is used to reflect ISO loudness contours (ISO 226) of normal hearing (NH) subjects. For example, a pre emphasis filter can be implemented using a high pass filter with a cut off frequency of 1200 Hz and an attenuation of 6 dB per octave, reducing signal amplitudes by about 18 dB for frequencies around 150 Hz. Since the pre emphasis filter is located before or is integrated within the channel specific band filters of the Pre-Processor Filter Bank 101, signal components are attenuated into a lower usable dynamic range in all the succeeding signal processing stages, resulting in a lower accuracy of the stimulation amplitude. FIG. 9 illustrates this loss of accuracy showing a logarithmic mapping using representative values of c=512, MCL=0.8 and THR=0.08. The dotted line in FIG. 9 shows the maximum upper input and output limit for a 150 Hz sinusoid attenuated by a pre emphasis filter. In this case, the maximum possible input signal ENVnorm is 0.126 and the maximum possible output signal ENVlog is 0.563. Signal ENVnorm and ENVlog correspond to normalised signal Sn and En of FIG. 1, respectively (n={1, . . . , N}). In the upper x-axis the corresponding sound pressure level in dB is shown (with no AGC).
Another disadvantage of using a high pass pre emphasis filter in the Pre-Processor Filter Bank 101 is that the sound level dependency of the ISO loudness contour will not be considered. For example, as shown in FIG. 10, an 89.5 phon contour is much shallower than a 35.5 phon contour and this cannot be modeled by high pass filtering (where the marks T and M also show minimum and maximum threshold levels for a 150 Hz sinusoid).
The Pulse Mapping Module 103 maps the stimulation event signal (typically signal envelope amplitudes) using a logarithmic map law function. This compensates for the exponential loudness growth of electrical stimulation. Since signal processing channels with low frequencies (e.g., <1200 Hz) are attenuated by the pre emphasis filter, the map law input signal cannot reach maximum amplitude, and in these attenuated channels consequently the most comfortable loudness (MCL) level also cannot be reached in the output electrode pulses. FIG. 9 shows this effect for a 150 Hz sinusoid, which corresponds to a low frequency band where the center frequency covers typical male and female F0. The attenuation before map law results in an unwanted reduction of the dynamic range of the corresponding signal and possibly in decreased hearing performance for CI listeners. Moreover, the resulting electric stimuli do not match to ISO loudness contours of NH subjects since only frequency-dependent and no amplitude-dependent attenuation is used.
As described above, the ISO loudness contours are roughly approximated by a high pass filter (pre emphasis filter). Instead of this high pass filter, a weighting of the channel specific band pass filter coefficients or a channel specific gain can be used. But any of these methods results in a reduction of the usable dynamic range (DR) in the succeeding signal processing stages. For example, after envelope extraction, amplitudes can be mapped by using:
                              ENV                      lo            ⁢                                                  ⁢            g                          =                                                            log                ⁡                                  (                                      1                    +                                          c                      ·                                              ENV                        norm                                                                              )                                                            log                ⁡                                  (                                      1                    +                    c                                    )                                                      ·                          (                              MCL                -                THR                            )                                +          THR                                    (                  Equ          .                                          ⁢          1                )            where ENVnorm represents the normalized envelope amplitude relative to the maximum possible envelope amplitude as obtained from the signal processing (proportional to sound pressure level of the input signal), and c represents the logarithmic mapping parameter. Minimum (THR) and a maximum (MCL) levels are the electrode-specific current levels. In FIG. 9, one specific example of the logarithmic mapping using Equation 1 is given where a 150 Hz sinusoid is used which corresponds to a high male fundamental frequency F0. Due to the pre emphasis filter, this signal can reach a maximum value of ENVlog=0.563 after mapping (indicated by the dotted line in FIG. 9). In this case, assuming MCL=0.8 and THR=0.08, the DR is 20*log10(0.563/0.08)=17 dB. Without a pre-emphasis filter the DR would be 20*log10(0.8/0.08)=20 dB. Thus the DR effectively is reduced by approximately 3 dB when a pre emphasis filter is used prior to the map-law stage.